BESIIIM. Anelli (Tecn.), R. Baldini Ferroli (ass.), M. Bertani (Resp.), A. Calcaterra, J. Dong (ass.),

G. Felici, S. Pacetti (Perugia), P. Patteri, D. Pierluigi (Tecn.),A. Russo (Tecn.), E. Tskhadadze (FAI fellowship) , Y. Wang (borsista), A. Zallo (ass.)

and in collaboration with:LNF Spas: S. Cerioni,LNF SEA: M. Gatta,

LNF Reparto Allineamento (Div. Acc.): M. Paris, F. PutinoINFN-Roma1 Servizio progettazione meccanica: A. Pelosi

1 The BESIII experiment

The BESIII experiment is successfully taking data since 2009 at the Beijing Electron PositronCollider BEPC-II, at the Institute of High Energy Physics, IHEP, and it will continue taking datauntil at least 2022. The BESIII detector is designed to study τ - charm physics and collected theworld largest samples of J/ψ, ψ(3686), ψ(3770), ψ(4040), Y (4260) and Y (4360). BESIII discoveredthe first unambiguous four-quark states, the charged Zc(3900)+ and Zc(4020)−.

The maximum instantaneous luminosity reached in 2014 is L = 0.8 × 1033 cm−2 s−1. In2014 the experiment has taken data for a precise R-scan of 104 energy points between 3.85 and4.59GeV, 1.0fb−1 at 4.42GeV, 0.1fb−1 at 4.47 GeV and 4.53 GeV for line shape, 0.04fb−1 at4.575 GeV (around the threshold of Λc), 0.5fb−1 at 4.60GeV.

In 2012 the LNF group started to work on the proposal of upgrading the inner BESIII trakingchamber, which is suffering early ageing due to the increase of the luminosity, with a new CylindricalGEM (CGEM) detector. The project has gone ahead and a Conceptual Design Report has beendrafted in 2014 with the contribution of the whole Italian BESIII group. The project, that since2014 also includes groups from Mainz, Uppsala and IHEP, has been recognized as a Great RelevanceProject within the Executive Program for Scientific and Technological Cooperation between Italyand P.R.C. for the years 2013-2015 and it has been selected as one of the projects funded by theEuropean Commission within the call H2020-MSCA-RISE-2014.The group is involved in the analysis of physics procesess mainly involving nucleons and lighthadrons.

2 Physics analysis

The INFN component of the BESIII Collaboration (BESIII-IT), represented by the LNF,INFN-Torino, INFN-Ferrara and INFN-Perugia groups, has contributed to the 2014 BESIII activ-ities in terms of proposals and data analyses concerning the following topics.

2.1 Measurement of the relative phase between strong and electromagnetic vector charmoniumdecay amplitudes

Two years ago BESIII-IT proposed an energy scan below, above and at the J/ψ to measurethe phase between strong and electromagnetic (EM) decay amplitudes, in a model independentway, from the interference between the J/ψ and the continuum for as much as possible processes.

It turned out that this phase, in modulus (i.e., apart the sign), is close to 90 degrees 1), at oddwith the 0 degree expectation. This result has also been achieved looking to the V P decays, where

V and P stand for vector and pseudoscalar mesons, in a model dependent way, as shown in Tab 1,

as well as looking to PP decays, updating the various J/ψ branching ratios 2).

Parameter FitSU3 strong amplitude g 7.22± 0.38SU3 breaking strange s 0.18± 0.04SU3 breaking OZI r −0.04± 0.02EM amplitude e 0.75± 0.04Relative phase φ (degree) 82± 7

Decay Amplitude PDG (10−4) PDG (10−4) ∆χ2

ρ0π0 geiφ + e 169.0± 15.0 133.00 1.13K∗+K− g(1− s)eiφ + e 51.2± 3.0 51.5 0.01K∗0K0 g(1− s)eiφ − 2e 43.9± 3.1 48.5 0.48ωη (gX + d)eiφ + eX 17.4± 2.0 18.5 0.06φη

[g(1− 2s)Y + d

]eiφ − 2eY 7.5± 0.8 3.9 4.02

ρη 3eX 1.9± 0.2 2.2 0.30ωπ 3e 4.5± 0.5 4.1 0.11ωη′ (gX ′ + d′)eiφ + eX ′ 7.0± 7.0 11.9 0.10φη′

[g(1− 2s)Y ′ + d′

]eiφ − 2eY ′ 4.0± 0.7 6.1 1.87

ρη′ 3eX ′ 1.1± 0.2 1.1 0.04

Table 1: Fit results for the SU3 parameterization of the J/ψ → V P decay rate.

In short, if the relative phase between strong and EM vector charmonium is different from zero,something important is lacking in the present charmonium description. We have put forward a

proposal to extend this measurement to the ψ(2S) 3).Perturbative QCD (pQCD) regime is supposed to be valid in the J/ψ decay, as also proven bythe small decay width. As a consequence, in such a regime, all the decay amplitudes, i.e., theamplitudes for any decay channel, are expected to be real. Accordingly a simple relationship isestablished between EM decay and continuum amplitude, which is supposed to be almost real aswell. On the other hand, no phase between decay amplitudes is foreseen in a description of vectorcharmonium by means of a Breit-Wigner (BW). In particular, unitarity would be violated in thecase of a simple BW description, if strong partial amplitudes would be imaginary. We have pro-

posed a model to explain this phase anomaly 4). Assuming this model the PANDA 5) experiment,for instance, making a scan of the total hadronic cross section should see the J/ψ as a dip instead

of a peak 6).The ψ(3770) width is greater than the accelerator energy spread. Such a meson decays mostlyin DD̄ and the aforementioned interference affects the peak value. In this way the phase can

be achieved and it turns out that it is close to −90 degrees 3). Conversely, present data onψ(2S) → V P decay indicate a phase close to 180 degrees, as shown in Tab. 2, while decay intoPP channels indicate a phase with modulus close to 90 degrees. Even ψ(2S)→ pp̄, nn̄ decays, as

measured by BESIII-IT 7), do not agree with the 90 degree found in the case of J/ψ (also measured

by the BESIII-IT 8)). As it has been anticipated, because of that, the BESIII-IT has put forward

a proposal for a ψ(2S) scan 3), which has been approved by the BESIII Collaboration and it willbe done hopefully during the 2015-2016 data taken.

Parameter FitSU3 strong amplitude g 0.49± 0.04SU3 breaking strange s −0.04± 0.13SU3 breaking OZI r −0.04± 0.08EM amplitude e 0.18± 0.02Relative phase φ (degree) 159± 12

Decay Amplitude PDG (10−4) PDG (10−4) ∆χ2

ρ0π0 geiφ + e 0.32± 0.13 0.28 0K∗+K− g(1− s)eiφ + e 0.17± 0.08 0.19 0K∗0K0 g(1− s)eiφ − 2e 1.09± 0.20 1.15 0.04ωη (gX + d)eiφ + eX 0.00± 0.11 0.08 0.06φη

[g(1− 2s)Y + d

]eiφ − 2eY 0.28± 0.90 0.24 0.10

ρη 3eX 0.22± 0.06 0.15 0.22ωπ 3e 0.11± 0.06 0.26 0.10ωη′ (gX ′ + d′)eiφ + eX ′ 0.32± 0.25 0.03 0.23φη′

[g(1− 2s)Y ′ + d′

]eiφ − 2eY ′ 0.31± 0.16 0.29 0.02

ρη′ 3eX ′ 0.19± 0.17 0.08 0.11

Table 2: Fit results for the SU3 parameterization of the ψ(2S)→ V P decay rate.

2.2 Measurement of the e+e− → Λ+c Λ−c cross section close to the production threshold

Many unexpected features has been observed, mostly by BaBar by means of the initial stateradiation technique, in the measurement of e+e− → pp̄ cross section close to the productionthreshold: a sudden jump in the total cross section, likely related to a Coulomb enhancement,

followed by a flat behavior and a form factor at threshold close to unity 10).The angular distribution is not isotropic, while isotropy at threshold is demanded by form factoranalyticity. In other words the electric and magnetic form factors, GE and GM , are not equal eachother at threshold. Unfortunately, this last result has been achieved integrating the cross sectionon a finite energy interval, to have enough statistics. Actually the interpretation of BaBar data is

controversial. A measurement of e+e− → Λ+c Λ−c cross section in the e+e−-center of mass can be

done even exactly at threshold, because of the weak decay into lighter particles, and BESIII cancheck and eventually confirm these features in the case of a heavy baryon. For this reason we have

proposed 9) to the BESIII Collaboration to perform the measurement of the e+e− → Λ+c Λ−c cross

section close to the production threshold and in part has been performed by BESIII at the end ofthe 2014 data taken.It can be anticipated that they have indeed been observed. As a consequence, first principlesassumptions, like analyticity, have to be revisited. Before claiming for these discoveries, we haveasked for a higher amount of integrated luminosity at the threshold, likely to be delivered in the

2015-2016 data taken 9).The measurement of baryon pair cross sections close to their thresholds, charged and neutral

(unexpected to be different from zero, as measured by BESIII 11)), emphasized by us 12) severaltimes, is driven the present BESIII data collection at low energies.

3 CGEM-IT related activities

The BESIII group has completed in 2014 the 2nd year of the Program of Great RelevancePGR00136, a 3-year joint project of INFN, IHEP, and the Italian Ministry of Foreign Affairs. Theobjective of this program, started in 2013 and ending with 2015, is a prototype of a cylindrical layerof detector, built with the technique of Cylindrical Gas Electron Multiplier (CGEM) developed inFrascati for the KLOE2 Inner Tracker.

This prototype layer has 2 roles: it will help the BESIII group to understand and test con-struction details differing from KLOE, and it will be finally incorporated, once proven functional,in the 3-layer Inner Tracker for the BESIII experiment at IHEP, whose Inner Drift Chamber isstarting to show signs of degradation by excessive backgrounds. This second requirement has com-plicated much the task, because the connections for all services, HV, gas, anchoring points, neededan ampunt of planning that usually may be overlooked in a prototype. In fig.1-top a sketch of theBESIII Inner Tracker around the BESIII beam pipe.

3.1 BESIII CGEM innovations

The experience achieved with the KLOE2 CGEM detector 21), 22) allows us to replicatewhat has worked best with KLOE2 and also to introduce new innovative concepts in the designand construction of the BESIII CGEM-IT detector.

The main innovation with respect to the KLOE2-IT is in the readout: for the KLOE-IT thereadout was digital, while for the BES3 CGEM-IT, destined to operate in a magnetic field twiceas high as the KLOE one, will be analogic. The goal is to be able to compute hit coordinatesby weighing the charge collected on adjacent anode strips, and thus to recover the resolutionlost because of the charge drift in the higher magnetic field. A new anode has been designedwith two-dimensional readout: X-strips, 570µm-wide parallel to the CGEM axis which provideρφ coordinates, and V-strips, 130µm-wide and with a stereo angle with respect to the X-strip,giving the z coordinate. The strip pitch is 650µm for both X- and V-strips, and the space betweenthe readout and the ground plane will be filled by a Rohacell structure. A ”jagged” configuration(fig.1-bottom) has been studied to minimize the inter-strip capacitance, in which the overlap regionbetween the strips is reduced by decreasing the size of the X-strips on the intersections. Additionalstudies on the anode are being studied with Garfield simulations and with tests on a small planarprototype. In order to achieve the required 100µm spatial resolution an analogue readout ASIC isbeeing designed by INFN-Turin investigating the UMC 0.11µm technology.

Moreover, a new Rohacell based tecnique, instead of standard Honeycomb, will be adopted to

manufacture the anode and the cathode electrodes in order to minimize the material budget 23).In 2014 large GEM foils needed for construction have been manufactured by the CERN TS-

DEM-PMT laboratory at CERN and delivered to LNF, here they have been tested, and identifieda few defects, that have been corrected at CERN. The fixed foils have been tested again in Frascati,and passed all selection criteria.

We have designed, ordered and received a new Al table, carrying precisely-drilled holes andholding fixtures for positioning and gluing 2 individual GEM foils of 3 different sizes into 3 biggerGEM foils, for subsequent wrapping in a cylindrical shape; the KLOE glueing table had dimensionsincompatible with BES3 cylinders.

We have designed, ordered and received the 5 cylindrical Al molds necessary for convertingthe glued planar GEM sheets into cylindrical ones. This has been done in the LNF clean roomwith a laser measuring system supervised by M. Paris, head of the LNF Alignment Department(Accelerator Division) assisted by F. Putino. This system (see Fig. 2) takes many measurementof points on the mold’s surface and fits them all to a cylindrical surface, giving RMS deviationsfrom it. All our molds had surfaces cylindrical within a few tens of microns. Starting from the

Figure 1: Top: longitudinal drawing of the BESIII three layers CGEM detector (green lines) alongthe BEPCII beam pipe,bottom: anode layout for XY strips with straight (a) and jagged (b) configuration.

beginning of 2015 the first complete CGEM layer will be constructed and by the end of the yearit will be tested with a provisional readout system.

Figure 2: The laser measurement of the 5 Al molds. The laser head is shown in front

3.2 Cosmic Ray Test Setup

The group has continued to run the cosmic ray telescope described in the 2013 ActivityReport, which is constituted by 2 scintillators, 4 small (10x10cm2) KLOE trackers, and a 10x10cm2

BESIII-like chamber, with the object of understanding as much as possible the properties of thenew type of anode sheets, made lately at the CERN workshop, that differ from the ones used forthe KLOE-IT. This apparatus, formerly read digitally with GASTONE electronics, was convertedin 2014 to use an analogical readout, based on the SRS system in use at CERN, developed in thecadre of the RD51 group of detector developers. This setup has the primary purpose to keep thehardware ready to operate at a test beam, although it can also be used for some measurement ofits own, with the limitations due to the unknown momenta of cosmic rays, and the systematicsdue to cosmic showers and multiple scattering.

3.3 CERN Test Beam

In December 2014 we have run a 3-week beam test at SPS H4 beam line at CERN, within theRD51 Collaboration, using the same apparatus tested in cosmic rays, with the main purpouses ofmeasuring efficiency at different gains, cluster size and resolution as function of the magnetic field,signal to noise ratio, test of the analog readout in magnetic field. The planar BESIII chamber hasbeen tested with two different gap sizes (3 and 5 mm) and two gas mixtures (Argon-CO2 70-30and Argon C2H10 90-10). The H4 beam line at the SPS has a secondary extracted beam of muonsand pions with momentum up to 400GeV/c, the facility has a dipole magnet (GOLIATH) capableof reaching B field values of 1.5T. A schematic drawing and a picture of the setup are shown infig.3 top part.

To exploit the features of the GEM detectors fully analog readout, both the tracking andthe test chambers have been instrumented by means of a front-end chain based on the APV25hybrid boards. These boards provide a 25 ns sampling of the input signals then allowing a coarse

Figure 3: top left: drawing of the Beam Test setup, particles going from left to right. The BESIIItest chamber (yellow) in-between the four trackers (green) the four trigger bars (blue). The wholesetup is placed in the magnet (purple). top right: picture of the whole setup inside the GOLIATHmagnet at Cern SPS H4 beam line. Bottom:(a) beam profile obtained from the BESIII testchamber, (b) x view of a single track without alignement.

reconstruction of the signal shape. The output data can be used for simple charge centroid mea-surements or, by means of a more complex analysis, to define the arrival time of the signals thenallowing to implement the so called µTPC readout. The µTPC method is very powerful as it al-lows to avoid the worsening of spatial resolution measurements carried out by means of the chargecentroid method in presence of strong (1 T) magnetic fields. The complete Test Beam chain wasmade of 12 APV boards (for a total amount of 768 channels) and a FADC-SRS card readoutthrough a GBD transceiver. The HV was provided by commercial CAEN system instrumentedwith CAEN A1550 power supply and distributed to the GEM electrodes by a custom distributionsystem. Singular current monitoring was provided by a nano-amperometer.

In fig.3 bottom part are shown the beam profile obtained by plotting all the clusters of theBESIII test chamber during one run (a) and the x-view of a reconstructed track, before alignement(b). Overall the data from this test beam are very satisfactory and its analysis just started.

References

1. BAM-128 Y. Wang et al., under review for publication on Phys. Rev. Lett.;BAM-106 M. Destefanis et al., under review for publication on Phys. Rev. Lett.

2. K. A. Olive et al. [Particle Data Group Collaboration], Chin. Phys. C 38 (2014) 090001.

3. BESIII-IT, BESIII Collaboration Meeting, Beijing, June 2014.

4. BESIII-IT, BESIII Physics and Software Meeting, Beijing, February 2013.

5. PANDA Experiment website: http://www-panda.gsi.de/.

6. BESIII-IT, PANDA Meeting, Frascati, September 2014.

7. BAM-126 K. Zhu et al., under review for publication on Phys. Rev. Lett..

8. M. Ablikim et al. [BESIII Collaboration], Phys. Rev. D 86 (2012) 032014.

9. BESIII-IT, BESIII Collaboration Meeting, Jinan, June 2014.

10. J. P. Lees et al., Phys. Rev. D 87 (2013) 092005.

11. X. Zhou et al., under review for publication on Phys. Rev. Lett..

12. BESIII-IT, BESIII Collaboration Meeting, Beijing, June 2013.

13. B. Aubert et al. [BaBar Collaboration] Phys. Rev. Lett. 95 (2005) 142001

14. C.Z. Yuan et al. [Belle Collaboration] Phys. Rev. Lett. 99 (2007) 182004

15. Z.Q.. Liu et al. [Belle Collaboration] Phys. Rev. Lett. 110 (2013) 252002

16. G. Lopez Castro, J. L. Lucio M. and J. Pestieau, AIP Conf. Proc. 342 (1995) 441.

17. R. Baldini, M. Bertani, C. Bini, R. Calabrese, A. De Falco, M. L. Ferrer, P. Gauzzi andE. Luppi et al., “Measurement of J/ψ → NN̄ branching ratios and estimate of the phase ofthe strong decay amplitude,” Phys. Lett. B 444 (1998) 111.

18. S. J. Brodsky and G. P. Lepage, Phys. Rev. D 24 (1981) 2848.

19. B. Aubert, et al. [BaBar Collaboration], Phys. Rev. D 76 (2007) 092006.

20. L. Yan, Report on BESIII Winter Collaboration Meeting 2013 ”ΛΛ at threshold”.

21. F.Archilli et al. [KLOE2 Collaboration], ”technical design report of the inner trackerfor theKLOE2 experiment” arXiv:1002.2572 (2010)

22. G.Morello, on behalf of the KLOE2-IT group, ”Construction and commissioning of the KLOE2Inner Tracker”, TIPP2014 proceeding (2014)

23. I. Garzia, on behalf of the BESIII CGEM-IT group, ”A cylindrical GEM detector with analogreadout for the BESIII experiment”, TIPP2014 proceeding (2014)